Support follow-up semantics can repopulate Unassigned

Candidate learning: In Shield support, a follow_up event parks an unresolved issue for six hours; it does not complete it. When the timer expires, the UI grouping routes that issue into Unassigned, and adding a note/contacted event also leaves the issue unresolved. A continuous all-pages board can therefore surface a large historical backlog that page 1 previously hid. Product-safe fix direction: distinguish “complete/handled” from “schedule another follow-up,” and exclude future parked follow-ups from the actionable queue or show them in a separate scheduled section. Provenance: web/src/lib/support-events.ts, web/src/lib/support-routing.ts, web/src/lib/issues/visible-issues.ts, web/src/components/super-admin/status/status-table.tsx; continuous-board change introduced in commit 04712085 and restored in 498855f4. No customer data included.